It says, "a couple of turns." I declare that a couple of turns is not enough.
Let's assume we have 200 Imps and 200 Familiars. In the first turn, the Imps steal 50 mana from the Necromancer and open a gate. In the first turn, the Familiars steal 25 mana and don't summon anyone. In the second turn, the Imps become an independent combat unit (by the way, they are not as vulnerable to general weakening as the Familiars). 80 Imps are summoned (with skillful gate opening). The Familiars steal another 25 mana (if they are not accidentally hit by Archliches or the Ring of Cold, or some other general plague, if they even survive, because Necromancers love to lower morale). In the third turn, the Imps are still fighting, if they survived, of course. The summoned Imps steal 20 mana (if they haven't been wiped out before the turn) and run into battle. The Familiars steal the same 25 mana, and they simply don't have time to summon anything.

In total, by the third turn, the Imps steal 70 mana, while also being able to fight. By the third turn, the Familiars will steal 75 mana if their number has not decreased. That is, for the sake of argument, in the third turn, you exchange 280 fighting Imps for 5 extra units of mana (and if the Necromancer feels the need to stop spending mana, he will simply blast the Familiars with bone arrows/Archliches/magic). In my opinion, it's not worth taking Familiars just to deprive the opponent of mana – they need to be protected, and it's not worth sacrificing a combat unit for dubious gain. By the way, it's unclear how you can determine in advance whether it will be possible for the Familiars to stand for several turns or not – it all depends on the Necromancer's desire. I can't imagine a battle in which the Necromancer allows his mana to be stolen for more than three turns if it's critical for him.

If there are a lot of Imps by chance (for example, creature dwellings, no losses of Imps against neutrals, joined troops), then due to their speed and with some randomly obtained divine power, the Familiars can be an excellent unit. Also, if the armies are not too large, and the Cave Lords can still change things with their magic, then the Familiars will be useful. In all other cases, Imps, which don't miss turns, are more reliable.

P.S. I didn't quite understand how the Familiars would prevent the Necromancer from using Spiritual Link? It gives a lot of mana with large armies, and the hero is only pushed back by half a turn. And if the armies are small, then the Familiars won't pump up much mana either.
Besides, you are only looking at Dark spells cast by the Necromancer. Has Chaos been officially banned? Necromancers quite like the Ring of Cold with cold perks.